This invention relates to a time/temperature monitoring and indicating device that indicates the length of time that such device has been exposed to temperatures above a predetermined temperature level. More particularly, the invention relates to a monitoring and indicating device for indicating by a dye-diffusion mechanism the length of time that frozen food packages, frozen food storage facilities and space or other temperature controlled products, devices or apparatus, has been exposed to undesirable temperature levels for critical periods of time. Utilizing this mechanism, the device graphically displays the storage and handling time/temperature history of frozen food packages subjected to any such thermal abuse.
In the past, many frozen food defrost indicators have been produced, but in general no or inadequate time/temperature history was provided. Among early attempts to provide time/temperature histories were some devices having time-delay intervals before triggering visible indicators. Usually such time delays were of very short duration, and consequently positive indications of excessive exposure to undesirable temperature levels were not significant and did not necessarily correlate with the quality of frozen foods offered. These devices largely were of the type which delayed delivery of a solvent to the indication site by various wicking techniques, including the wicking of solvent through a constricted passageway. Time delays afforded thereby were on the order of several minutes to a few hours.
The requirements for such time/temperature history indicators arises as each package or case-lot of frozen food presents a complex physical/chemical system that goes through subtle changes upon exposure to excessive temperature levels before becoming unusable. In some frozen foods, the changes are reversible as they represent for example, no enzymatic degradation or crystal restructuring upon refreezing. In some foods, notably beef, shellfish and ice cream, flavor and texture are affected by undesirable temperature levels and such changes are irreversible.
To reduce the risk of unwittingly selling spoiled frozen foods, various indicators have been developed which yield the respective history of refrigeration or the intactness of the cold temperature storage, so far as may be indicated by deformation or by changes in color. However, such indicators frequently have been insufficient in the time aspect as some foods experience subtle changes only after several days at a specific temperature level.
Some chemically or physically reacting indicators are known which indicate any change in the condition of the goods resulting from temperature rises by the indicators changing their shape or color or by bursting. The known indicators have the additional disadvantage that their manufacture, storage, or application to frozen foods is too complicated, or that the readings provided thereby are not sufficiently reliable, or that they cannot be manufactured at a price in keeping with the value of the goods to be protected.
The main advantage of the device of this invention is that a time/temperature indication system is provided whereby an indication of time at a predetermined temperature is obtained by a unique dye-diffusion mechanism which permits the measurement of time periods, above a predetermined critical temperature, of days or weeks, instead of minutes or hours, as will be described in more detail below.